A CRT electron gun for controlling divergence angle of electron beams according to intensity of current

Abstract: This invention relates to an inline electron gun for a color cathode ray tube, more particularly to an inline electron gun which can provide high resolution by controlling intensity of electrostatic lenses that controls electron beams according to intensity of current, dynamically. The electron gun for a cathode ray tube includes a three electrode part having a part formed of a plurality of inline electron beam emitting means for emitting electron beams and the other part formed of control electrodes and acceleration electrodes for controlling quantity of the emission and forming a crossover of the electron beams, a plurality of focusing electrodes and positive electrodes forming a main electrostatic focusing lenses for focusing the electron beam onto a screen, the electron beam emitting means and the plurality of electrodes are aligned in line with the tube axis spaced in a certain interval, successively, and a supplementary electrode having a fixed thickness and synchronizing to application signal of the electron beam emitting means, the supplementary electrode is positioned between the acceleration electrode and the focusing electrode adjacent to the acceleration electrode and is for forming an enlargement electrostatic lens for controlling the divergence angle of the electron beam according to the intensity of current.

Description: FIELD OF THE INVENTION This invention relates to an inline electron gun for a color cathode ray tube, more particularly to an inline electron gun which can provide high resolution by dynamically controlling the electrostatic lenses that control electron beams according to the intensity of beam current. DESCRIPTION OF THE PRIOR ART As shown in FIG. 1, a prior art electron gun includes a cathode 1 for discharging an electron beam, a control electrode 2 for controlling the discharge of electrons, a first acceleration electrode 3 for accelerating the electron beam toward a screen, and an arrangement of many electrodes having at least two focusing lenses 4 and a second acceleration electrode 5. The control electrode 2 is generally grounded and the first acceleration electrode 3 has a voltage of 300 to 1000V applied thereto to form an electron beam forming region. The focusing electrodes 4 and the second acceleration electrode 5, have a focusing voltage (Vf ; 5000 to 9000 V) and an accelerating voltage (Vg ; 20000 to 32000V) applied thereto to form a main electrostatic lens 9. The electron beam emitted from the cathode 1 starts to diverge at a crossover 7 near the first acceleration electrode 3. The diverging electron beam is focused primarily by the electrostatic lens formed of a potential difference between the first acceleration electrode 3 and the focusing electrode 4, i.e., by the prefocusing lens 8, and forms a small pixel on the screen 6 by virtue of the main electrostatic lens 9. In the foregoing prior art electron gun, electrons are controlled by a cathode voltage Vc which has an amplitude which varies as a function of time and is discharged through the cathode 1 as heat, while the intensity of current is determined by the configuration of the electron beam forming regions and the voltages applied to the cathode and various electrodes. Accordingly, the electrons form a crossover 7 and start to diverge therefrom. The diverging electrons are focused primarily by the prefocusing lens formed of the potential difference between the first acceleration electrode 3 and the focusing electrode 4 and, again by the main electrostatic lens 9 to form a small beam spot on the screen. The beam, i.e., spot, size has a very close relationship to the resolution of a CRT, and in general as the resolution is improved the smaller the spot size becomes. Major factors affecting the spot size are the arrangement of the main lens, spherical astigmation, and a space charge exerting force on the electrons in the free space between the electron gun and the screen. These factors have a very close relationship with the divergence angle of an electron beam incident on the main electrostatic lens 9 at a fixed angle shown in FIG. 2. That is, the spot size Dx due to the main lens magnification M has a relation Dx=Mdx to a size dx of a virtual image 11 obtained by extrapolation of the divergence angle, and the spot size Dsa due to the spherical astigmation has a relation Dsa=Csα 3 to a spherical astigmation coefficient Cs and a divergence angle α. The spot size Dsc due to the space charge in connection with a current intensity I can be expressed as Dsc = 0.8βIL 2 /DbVa 3 2 where Db is the thickness of the beam in the main lens, L is the distance from the main lens to the screen, Va is the maximum voltage at a positive pole, and β is a constant for electrons moving in a vacuum electric field. The overall spot size Dt determined by the above equations can be expressed as Dt=√ ((Dx + Dsc) 2 + Dsa 2 ) and the divergence angle at which the overall spot size is the smallest is called the optimum divergence angle α o . Shown in FIG. 3 is the optimum divergence angle α o , where the abscissa and the ordinate represent the divergence angle α and the spot size D, respectively. FIG. 3 shows that the overall spot size Dt is determined by the spot size Dx due to the arrangement of the main lens, the spot size Dsa due to the spherical astigmation, and the spot size Dsc due to the space charge, and that the divergence angle of the abscissa at which the spot size is the smallest is the optimum divergence angle α o . FIG. 4 is a graph showing the optimum divergence angle as a function of the intensity of current, where optimum divergence angles α o1 , α o2 and α o3 at various intensities of current 1mA, 2mA and 3mA can be seen as well as increases in the spot sizes Do 1 , Do 2 , Do 3 according to increases in the intensity of current 1mA, 2mA and 3mA. FIG. 5 shows the change of divergence angle Ao and the change of optimum divergence angle Bo as a function of intensity of current in a prior art electron gun, where it can be seen that the deviation in the divergence angle from the optimum divergence angle becomes greater at higher intensities of current over 1mA for the electrons of a prior art electron gun resulting in degradation of spot size. Thus, when an electron gun has been designed to an optimum divergence angle at a specific intensity of current, the electron gun does not work at an optimum angle at other intensities of current. Therefore, because the divergence angle increases very sharply as the intensity of current increases in the prior art electron gun, the magnification of the main lens should be changed to compensate for the change in intensity of current. Consequently, because the voltage of the focusing electrode has had to be changed in accordance with the intensity of current for the foregoing reasons, the prior art electron gun has had problems associated with high production cost and the requirement of additional electrodes. One attempt at providing a solution to this problem is described in US-A-4,334,170 in which there is disclosed an electron gun specifically designed to provide optimum beam spot size. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a device comprising a cathode ray tube and a voltage applying means, said cathode ray tube comprising an electron gun comprising: a three electrode part formed of a plurality of mutually aligned electron beam emitting means, control electrodes and acceleration electrodes, the electron beam emitting means emitting an electron beam and the control electrodes and the acceleration electrodes controlling the quantity of the emitted electrons and forming a crossover in the electron beam; a plurality of focusing electrodes and accelerating electrodes forming a main electrostatic focusing lens for focusing the electron beam onto a screen, said electron beam emitting means and said plurality of electrodes being aligned with an axis of the cathode ray tube and spaced successively at predetermined intervals; and, a supplementary electrode having a predetermined thickness positioned between the acceleration electrodes and the focusing electrodes adjacent to the acceleration electrodes, characterised in that in use the supplementary electrode has a voltage applied thereto from said voltage applying means which is synchronized with that which is applied to said electron beam emitting means, said supplementary electrode forming an enlargement electrostatic lens for controlling the divergence angle of the electron beam according to the intensity of the beam current. According to a second aspect of the present invention there is provided a device comprising a cathode ray tube and a voltage applying means, said cathode ray tube comprising an electron gun comprising: a three electrode part formed of a plurality of mutually aligned electron beam emitting means, control electrodes and acceleration electrodes, the electron beam emitting means emitting an electron beam and the control electrodes and the acceleration electrodes controlling the quantity of the emitted electrons and forming a crossover in the electron beam; a plurality of focusing electrodes and accelerating electrodes forming a main electrostatic focusing lens for focusing the electron beam onto a screen, said electron beam emitting means and said plurality of electrodes being aligned with an axis of the cathode ray tube and spaced successively at predetermined intervals; and a supplementary electrode having a predetermined thickness, characterised in that said plurality of focusing electrodes has a first focusing electrode and a second focusing electrode adjacent the accelerating electrodes to which in use the same voltage is applied from said voltage applying means; and said supplementary electrode is positioned between the first focusing electrode and the second focusing electrode, the supplementary electrode having a voltage applied thereto which is synchronized with that which is applied to said electron beam emitting means. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-section of part of a prior art electron gun. FIG. 2 shows a lens of the prior art electron gun. FIG. 3 shows a general way of calculating an optimum divergence angle for a given spot size. FIG. 4 shows a general way of calculating an optimum angle as function of intensity of current. FIG. 5 shows the divergence angle and the optimum divergence angle as a function of intensity of current for the prior art electron gun. FIG. 6 is a schematic cross-section of part of an electron gun in accordance with an embodiment of the present invention. FIG. 7 is a schematic cross-section of an electron gun in accordance with other embodiments of the present invention. FIGs. 8a to 8c are detailed views of a cathode dynamic electrode of an electron gun in accordance with an embodiment of the present invention, wherein FIG. 8a is a plan view, FIG. 8b is a cross-sectional view taken along line A-A' of FIG. 8a, and FIG.8c is a cross-sectional view taken along line B-B' of FIG. 8a. FIG. 9 shows voltage wave patterns as a function of time applied to the cathode dynamic electrode of FIGs. 8a to 8c. FIG. 10a and 10b are comparison graphs as a function of intensity of current when the cathode dynamic electrode of FIGs. 8a to 8c is inserted between a first acceleration electrode and a focusing electrode, wherein FIG. 10a is a comparison of divergence angle as a function of intensity of current, and FIG. 10b is a comparison of spot size as a function of intensity of current. FIGs.11a and 11b are comparison graphs as a function of intensity of current when the cathode dynamic electrode of FIGs 8a to 8c is inserted between a first focusing electrode and a second focusing electrode, wherein FIG. 11a is a comparison of divergence angle as a function of intensity of current, and FIG. 11b is a comparison of spot size as a function of intensity of current. DETAILED DESCRIPTION OF THE EMBODIMENT This invention is to be explained in detail hereinafter, referring to the attached drawings. FIG. 6 is a cross-sectional view of part of an electron gun in accordance with an embodiment of this invention. The electron gun includes a cathode 1 for emitting a beam of electrons, a plurality of electrodes 2 to 5 for controlling, accelerating and focusing the emitted electron beam, and a supplementary electrode, i.e., a cathode dynamic electrode, 20 of a predetermined thickness tl which is inserted between a first acceleration electrode 3 and a focusing electrode 4 and has an aperture 11. The electron gun is operated by applying a cathode dynamic voltage Vc' amplified through synchronizing to a cathode voltage Vc which has an amplitude (amplitude of voltage) which varies as a function of time from the cathode dynamic electrode 20. FIG. 7 is an electron gun in accordance with another embodiment of this invention, wherein the electron gun includes a cathode dynamic electrode 20' of a predetermined thickness t2 inserted between a first focusing electrode 4A and a second focusing electrode 4B and having an aperture 12. Once again the electron gun is operated by applying a cathode dynamic voltage Vc' amplified through synchronizing to a cathode voltage Vc which has an amplitude (amplitude of voltage) which varies as a function of time from a cathode dynamic electrode 20'. FIGs. 8a to 8c are detailed views of the cathode dynamic electrode of an electron gun embodying this invention, wherein, as shown in FIG. 8a, a space between the rims of three holes 21R, 21G and 21B and the rim of the electrode 22 is treated with a ceramic metallizing treatment process to form a ceramic insulation part 23 (hatched part) for electrically insulating the three holes 21R, 21G and 21B. One lead 24 (dotted lines) is, buried in the ceramic part for insulating the lead line from others provided to each of the three holes 21R, 21G and 21B to apply power to the three holes 21R, 21G and 21B. To fix the cathode dynamic electrode described above to the electron gun in the middle between two electrodes, a plurality of bead glass insertion parts 25 are provided at top and bottom aligned with the centres of each of the holes 21R, 21G and 21B as shown in FIG. 8a. The insertion parts 25 project a distance h from the rim 22 of the electrode as shown in FIG. 8c. The operation and advantages of an electron gun having the foregoing construction will now be explained. As shown in FIG. 9, the cathode voltage Vc which has an amplitude (voltage amplitude) which varies as a function of time is amplified as B Vc', and synchronized to the cathode dynamic electrode 20 inserted between the first acceleration electrode 3 and the focusing electrode 4 shown in FIG. 6. Accordingly, there is formed an enlargement electrostatic lens 8' which is controllable and sensitive as a function of intensity of current position between the first acceleration electrode 3 and the focusing electrode 4. As shown in FIG. 10a, when the intensity of current is high, the potential of the cathode is in general low and, since the potential of the cathode dynamic electrode 20 is synchronized thereto becomes also lower subsequently. In addition the enlargement electrostatic lens 8' becomes thicker and the divergence angle α of the electron beam becomes smaller. By contrast, when the intensity of current is low, the cathode potential becomes higher and, since the potential of the cathode dynamic electrode 20 is synchronized thereto becomes also higher subsequently. The enlargement electrostatic lens 8' becomes thinner and the divergence angle α of the electron beam becomes greater. In conclusion, a graph α 2 of the change in the divergence angle can be formed which exhibits comparatively less variation compared to a graph α 1 of the change in the divergence angle of the prior art electron beam. As shown in FIG. 10b, because of the function of the enlargement electrostatic lens 8' described above in enabling electron beams to be directed to the main lens 9 with an optimum divergence angle for a given intensity of current, a graph D 2 of the change in spot size exhibits comparatively less variation compared to a graph D 1 of the change in spot size of the prior art electron beam. Therefore, by facilitating an appropriate control of the spot size small dense pixels can be reproduced thereby affecting the resolution of a CRT. In addition, in the prior art electron gun having another electrode inserted between the first focusing electrode and the second focusing electrode, a fourth grid and the first acceleration electrode 3 have the same potential. Together they form a unipotential lens between the first focusing electrode and the second focusing electrode to focus the electron beam in a multi-stage process in which the beam is primarily focused at the prefocusing lens 8. The beam is directed toward the main lens 30, thereby reducing astigmation to a minimum. However, in the embodiment of this invention shown in FIG. 7, the voltage Vc' applied to the cathode dynamic electrode 20' is synchronized to the cathode voltage Vc and amplified. Accordingly, as shown in FIG. 11a, when the intensity of current is high, since the voltage of the cathode dynamic electrode 20' synchronized thereto becomes lower, the enlargement electrostatic lens 8" becomes thicker and the divergence angle a of the electron beam is reduced. In addition, when the intensity of current is low, since the voltage of the cathode dynamic electrode 20' synchronized thereto becomes higher, the enlargement electrostatic lens 8" becomes thinner and the divergence angle α of the electron beam is increased. Thus a graph α 4 exhibiting comparatively less variation in divergence angle compared to a graph of the divergence angle α 3 of the prior art electron beam can be formed. As shown in FIG. 11b, because of the function of the enlargement electrostatic lens 8" (forward focusing lens) in directing electron beams to the main lens with an optimum divergence angle for a given intensity of current, a graph of change in spot size D4 exhibits comparatively less variation compared to a graph D3 of the change of spot size of a prior art electron beam. As a result, this invention facilitates an appropriate control of the spot size which affects the resolution of a CRT and enables the reproduction of small dense pixels. As has been explained, since this invention can provide spots which exhibit little change in size, and in particular can provide spots which become smaller at high intensity of current, this invention has the advantage of improving the resolution of color picture tubes. Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the scope of the appended claims.

Claims: A device comprising a cathode ray tube and a voltage applying means, said cathode ray tube comprising an electron gun comprising: a three electrode part formed of a plurality of mutually aligned electron beam emitting means (1), control electrodes (2) and acceleration electrodes (3), the electron beam emitting means (1) emitting an electron beam and the control electrodes (2) and the acceleration electrodes (3) controlling the quantity of the emitted electrons and forming a crossover (7) in the electron beam; a plurality of focusing electrodes (4) and accelerating electrodes (5) forming a main electrostatic focusing lens (9) for focusing the electron beam onto a screen (6), said electron beam emitting means (1) and said plurality of electrodes (4,5) being aligned with an axis of the cathode ray tube and spaced successively at predetermined intervals; and, a supplementary electrode (20) having a predetermined thickness positioned between the acceleration electrodes (3) and the focusing electrodes (4) adjacent to the acceleration electrodes (3), characterised in that in use the supplementary electrode (20) has a voltage applied thereto from said voltage applying means which is synchronized with that which is applied to said electron beam emitting means (1), said supplementary electrode (20) forming an enlargement electrostatic lens (8') for controlling the divergence angle of the electron beam according to the intensity of the beam current. The device as claimed in claim 1, wherein the supplementary electrode (20) has a tube shape and includes three electron beam pass through holes (21R, 21G, 21B) arranged inline for passing electron beams, an insulation part (23) formed between rims of the holes (21R, 21G, 21B) and a rim (22) of the electrode (20) for insulating said rims, and leads (24) inserted in the insulation part (23) for supplying power to the rim of each of the holes (21R, 21G, 21B). A device comprising a cathode ray tube and a voltage applying means, said cathode ray tube comprising an electron gun comprising: a three electrode part formed of a plurality of mutually aligned electron beam emitting means (1), control electrodes (2) and acceleration electrodes (3), the electron beam emitting means (1) emitting an electron beam and the control electrodes (2) and the acceleration electrodes (3) controlling the quantity of the emitted electrons and forming a crossover (7) in the electron beam; a plurality of focusing electrodes (4) and accelerating electrodes (5) forming a main electrostatic focusing lens (9) for focusing the electron beam onto a screen (6), said electron beam emitting means (1) and said plurality of electrodes (4,5) being aligned with an axis of the cathode ray tube and spaced successively at predetermined intervals; and a supplementary electrode (20) having a predetermined thickness, characterised in that said plurality of focusing electrodes (4) has a first focusing electrode (4A) and a second focusing electrode (4B) adjacent the accelerating electrodes (5) to which in use the same voltage is applied from said voltage applying means; and said supplementary electrode (20) is positioned between the first focusing electrode (4A) and the second focusing electrode (4B), the supplementary electrode (20) having a voltage applied thereto which is synchronized with that which is applied to said electron beam emitting means (1). The device as claimed in claim 3, wherein the supplementary electrode (20) has a tube shape and includes three electron beam pass through holes (21R, 21G, 21B) arranged inline for passing electron beams, an insulation part (23) formed between rims of the holes (21R, 21G, 21B) and a rim (22) of the electrode (20) for insulating said rims, and leads (24) inserted in the insulation part (23) for supplying power to the rim of each of the holes (21R, 21G, 21B).

Inventors: CHOI JIN YEAL, PARK MYUNG HO, CHOI, JIN YEAL, PARK, MYUNG HO
Applicants: LG ELECTRONICS INC, LG ELECTRONICS INC.
Application year: 1994
Main classification: H01J29
IPC codes: H01J29